As a mechanism installed in an internal combustion engine of a vehicle, a variable valve timing mechanism that allows valve timing of engine valves (intake and exhaust valves) to be varied, as described in Patent Documents 1 and 2, has been known. As a variable valve timing mechanism that has been put into practical use, there is a hydraulic mechanism that operates based on hydraulic pressure, as described in Patent Document 1.
As shown in FIG. 6, in the hydraulic variable valve timing mechanism 1, a vane rotor 3 is fixed to a camshaft 2 to rotate integrally with the camshaft 2. A substantially annular housing 5 is disposed about the outer circumference of the vane rotor 3 to be rotatable relative to the vane rotor 3. The housing 5 is fixed to a cam sprocket 4 to integrally rotate with the cam sprocket 4. On the outer circumference of the vane rotor 3, a plurality of vanes 6 are formed so as to project along the radial direction. Each vane 6 is housed in each of recessed portions 7, the number of which is equal to that of the vanes 6, formed on the inner circumference of the housing 5.
Inside each recessed portion 7, two oil chambers are defined by a vane 6. Of these, the oil chamber formed in the camshaft rotation direction of the vane 6 is a retarding oil chamber 8 in which hydraulic pressure is introduced to retard valve timing. The oil chamber formed in the camshaft counter-rotation direction of the vane 6 is an advancing oil chamber 9 in which hydraulic pressure is introduced to advance valve timing. The hydraulic pressure in the retarding oil chamber 8 and the hydraulic pressure in the advancing oil chamber 9 are adjusted by an oil control valve (OCV) 11 controlled by an electronic control unit (ECU) 10 for engine control.
Such a hydraulic variable valve timing mechanism includes a mechanical lock mechanism that maintains valve timing when the engine is started with insufficient hydraulic pressure. The lock mechanism is formed by a lock pin 12 slidably disposed with one of the vanes 6 of the vane rotor 3 and a lock hole 13 formed in the cam sprocket 4. The lock pin 12 can be fitted in the lock hole 13. The lock pin 12 is biased in the direction to be fitted in the lock hole 13 by a spring 14 provided at the proximal end of the lock pin 12. To the lock pin 12, hydraulic pressure is applied so as to resist the biasing force of the spring 14 in response to supply of hydraulic pressure to the retarding oil chamber 8 or the advancing oil chamber 9. As such a mechanical lock mechanism, there is also known a configuration in which a lock pin and a lock hole are provided on the radially inner circumference of the housing and on the radially outer circumference of the vane.
In the hydraulic variable valve timing mechanism 1 shown in the same drawing, the lock pin 12 and the lock hole 13 are disposed so that they are aligned when the vane rotor 3 rotates relative to the housing 5 to the maximum setting in the retarding direction (in the counter-rotation direction of the camshaft 2). There is also known a hydraulic variable valve timing mechanism that executes locking at the most advancing phase, at which a vane rotor 3 rotates to the maximum setting in the rotation direction of the camshaft 2 relative to the housing 5, and a hydraulic variable valve mechanism that executes locking in an intermediate locking phase between the most advancing phase and the most retarding phase.
In the hydraulic variable valve timing mechanism having such a lock mechanism, hydraulic pressure is supplied to the retarding oil chamber 8 and the advancing oil chamber 9 after the engine is started. After the lock pin 12 is released (disengaged from the lock hole 13), relative rotation of the vane rotor 3 with respect to the housing 5, that is, change of a valve timing, is started.